Method for forming a pcram with low reset current

ABSTRACT

Phase-change memory structures are formed with ultra-thin heater liners and ultra-thin phase-change layers, thereby increasing heating capacities and lowering reset currents. Embodiments include forming a first interlayer dielectric (ILD) over a bottom electrode, removing a portion of the first ILD, forming a cell area, forming a u-shaped heater liner within the cell area, forming an interlayer dielectric structure within the u-shaped heater liner, the interlayer dielectric structure including a protruding portion extending above a top surface of the first ILD, forming a phase-change layer on side surfaces of the protruding portion and/or on the first ILD surrounding the protruding portion, and forming a dielectric spacer surrounding the protruding portion.

TECHNICAL FIELD

The present disclosure relates to phase-change memory structures(PCRAMs) for non-volatile memory (NVM) devices. The present disclosureis particularly applicable to a PCRAM with a high heating capacity at alow reset current.

BACKGROUND

PCRAM structures have received increased attention because of theirsimplicity and scalability as compared with floating-gate (FG) andnitride-based NVM devices. However, PCRAM structures face severalchallenges. For example, a high current is required to generate the heatquickly and at a high magnitude to promote the fast phase-change(resistance change) of the phase-change material.

A need therefore exists for methodology enabling PCRAM structures withhigher heating capacities at lower reset currents, and the resultingstructures.

SUMMARY

An aspect of the present disclosure is an efficient method forfabricating a PCRAM structure with a low reset current and a highheating capacity having an ultra-thin heater liner, an ultra-thinphase-change layer, and a small contact area between the heater linerand the phase-change layer.

Another aspect of the present disclosure is a PCRAM structure with a lowreset current and a high heating capacity having an ultra-thin heaterliner, an ultra-thin phase-change layer, and a small contact areabetween the heater liner and the phase-change layer.

Additional aspects and other features of the present disclosure will beset forth in the description which follows and in part will be apparentto those having ordinary skill in the art upon examination of thefollowing or may be learned from the practice of the present disclosure.The advantages of the present disclosure may be realized and obtained asparticularly pointed out in the appended claims.

According to the present disclosure, some technical effects may beachieved in part by a method including: forming a first interlayerdielectric (ILD) over a bottom electrode, removing a portion of thefirst ILD, forming a cell area, forming a u-shaped heater liner withinthe cell area, forming an interlayer dielectric structure within theu-shaped heater liner, the interlayer dielectric structure including aprotruding portion extending above a top surface of the first ILD,forming a phase-change layer on side surfaces of the protruding portionand/or on the first ILD surrounding the protruding portion, and forminga dielectric spacer surrounding the protruding portion.

Aspects of the present disclosure include forming the phase-change layeron the first ILD, and removing a portion of the phase-change layervertically contiguous with the portion of the first ILD, prior toforming the u-shaped heater liner. Another aspect includes forming theu-shaped heater liner within the cell area and between the phase-changelayer and the protruding portion with a top surface of the u-shapedheater liner being coplanar with a top surface of the phase-changelayer. Further aspects include forming the phase-change layer on thefirst ILD, forming the dielectric spacer on the phase-change layer, andremoving a portion of the phase-change layer from the first ILD notunder the dielectric spacer. Additional aspects include forming a secondILD over the phase-change layer, and forming the protruding portion to aheight that is coplanar with a top surface of the second ILD. Otheraspects include forming a second ILD over the first ILD prior to formingthe phase-change layer, removing a portion of the second ILD verticallycontiguous with the portion of the first ILD, prior to forming theu-shaped heater liner. Further aspects include forming the phase-changelayer on side surfaces of the protruding portion, and forming thedielectric spacer surrounding the phase-change layer. Additional aspectsinclude forming the phase-change layer on side surfaces of theprotruding portion and on the first ILD, and forming the dielectricspacer above the phase-change layer on the first ILD and surrounding thephase-change layer on the side surfaces of the protruding portion. Afurther aspect includes forming a top electrode over the first ILD, thedielectric spacer, and the protruding portion.

Another aspect of the present disclosure is a device including: a bottomelectrode, a u-shaped heater liner, an ILD on the bottom electrode,surrounding the u-shaped heater liner, an interlayer dielectricstructure surrounded by the u-shaped heater liner and including aprotrusion extending above the u-shaped heater liner and a top surfaceof the ILD, a phase-change layer on the ILD and surrounding theinterlayer dielectric structure and/or on side surfaces of theprotrusion, a dielectric spacer above the ILD and surrounding theprotrusion, and a top electrode covering the ILD, the protrusion, andthe dielectric spacer.

Aspects include a device including the phase-change layer being on theILD under the dielectric spacer. Further aspects include a top surfaceof the phase-change layer being level with a top surface of an exposedportion of the u-shaped heater liner above the ILD, and the phase-changelayer surrounding the exposed portion of the u-shaped heater liner.Another aspect includes the phase-change layer being on side surfaces ofthe protrusion, between the dielectric spacer and the protrusion. Afurther aspect includes the phase-change layer being contiguous andaligned with vertical portions of the u-shaped heater liner. Yet anotheraspect includes a height of the phase-change layer and a height of theprotrusion being substantially the same. A further aspect includes thetop surface of the phase-change layer extending above a top surface ofthe dielectric spacer. Another aspect includes a thickness of a heaterliner layer forming the u-shaped heater liner being the same as athickness of the phase-change layer. Additional aspects include a firstportion of the phase-change layer being on the ILD under the dielectricspacer, and a second portion of the phase-change layer being on sidesurfaces of the protrusion between the dielectric spacer and theprotrusion. An additional aspect includes the first portion of thephase-change layer being contiguous and aligned with vertical portionsof the u-shaped heater liner.

Another aspect of the present disclosure includes: forming a first ILDover a bottom electrode, forming a second ILD on the first ILD, removinga portion of the first and second ILDs, forming a cavity, conformallydepositing a heater liner over the second ILD and within the cavity,planarizing the heater liner down to the second ILD, filling the cavitywith an interlayer dielectric material, removing a portion of the heaterliner adjacent the second ILD and the second ILD, leaving a portion ofthe interlayer dielectric material protruding from the cavity, forming aphase-change layer on side surfaces of the protruding portion and/or onthe first ILD surrounding the protruding portion, forming a dielectricspacer surrounding the protruding portion, and forming a top electrodeover the first ILD, the dielectric spacer, and the protruding portion.

Additional aspects and technical effects of the present disclosure willbecome readily apparent to those skilled in the art from the followingdetailed description wherein embodiments of the present disclosure aredescribed simply by way of illustration of the best mode contemplated tocarry out the present disclosure. As will be realized, the presentdisclosure is capable of other and different embodiments, and itsseveral details are capable of modifications in various obviousrespects, all without departing from the present disclosure.Accordingly, the drawings and description are to be regarded asillustrative in nature, and not as restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is illustrated by way of example, and not by wayof limitation, in the figures of the accompanying drawings and in whichlike reference numerals refer to similar elements and in which:

FIGS. 1 through 11 schematically illustrate a method for forming a PCRAMstructure with a low reset current, in accordance with an exemplaryembodiment; and

FIGS. 12 through 21 schematically illustrate alternative methods forforming alternative PCRAM structures with low reset currents, inaccordance with other exemplary embodiments.

DETAILED DESCRIPTION

In the following description, for the purposes of explanation, numerousspecific details are set forth in order to provide a thoroughunderstanding of exemplary embodiments. It should be apparent, however,that exemplary embodiments may be practiced without these specificdetails or with an equivalent arrangement. In other instances,well-known structures and devices are shown in block diagram form inorder to avoid unnecessarily obscuring exemplary embodiments. Inaddition, unless otherwise indicated, all numbers expressing quantities,ratios, and numerical properties of ingredients, reaction conditions,and so forth used in the specification and claims are to be understoodas being modified in all instances by the term “about.”

The present disclosure addresses and solves the current problem of highcurrent requirements to generate heat in a short amount of time and at ahigh magnitude attendant upon promoting a fast phase-change in aphase-change layer in a PCRAM. In accordance with embodiments of thepresent disclosure, a PCRAM structure is produced with an ultra-thinheater liner, an ultra-thin phase-change layer, and a small contact areabetween the heater liner and the phase-change layer, resulting in ahigher heating capacity at a lower reset current.

Methodology in accordance with embodiments of the present disclosureincludes forming an ILD over a bottom electrode, removing a portion ofthe ILD, forming a cell area, and forming a u-shaped heater liner withinthe cell area followed by forming an interlayer dielectric structurewithin the u-shaped heater liner. The interlayer dielectric structuremay include a protruding portion extending above a top surface of theILD. A phase-change layer may then be formed on side surfaces of theprotruding portion and/or on the ILD surrounding the protruding portion.Then, a dielectric spacer may be formed surrounding the protrudingportion. After forming the dielectric spacer, a top electrode may beformed over the ILD, the dielectric spacer, and the protruding portion.

Still other aspects, features, and technical effects will be readilyapparent to those skilled in this art from the following detaileddescription, wherein preferred embodiments are shown and described,simply by way of illustration of the best mode contemplated. Thedisclosure is capable of other and different embodiments, and itsseveral details are capable of modifications in various obviousrespects. Accordingly, the drawings and description are to be regardedas illustrative in nature, and not as restrictive.

Adverting to FIG. 1, a method of forming a PCRAM with a low resetcurrent, in accordance with an exemplary embodiment, begins with anelectrode 101. The electrode 101 may be formed of aluminum (Al),platinum (Pt), titanium nitride (TiN), titanium nitride and titanium(TiN/Ti), ruthenium (Ru), nickel (Ni), or polysilicon.

Next, a first ILD 201 may be formed over the electrode 101, asillustrated in FIG. 2. The first ILD 201 may be formed of a low thermalconductivity dielectric, such as silicon oxide (SiO₂), siliconoxynitride (SiON), silicon nitride (Si₃N₄), or fluorine doped SiO₂.

As illustrated in FIG. 3, a phase-change layer 301 may be formed overthe first ILD 201. The phase-change layer 301 may be formed of any typeof phase-change material, such as GST (Ge₂Sb₂Te₅) or AIST (silver andindium doped Sb₂Te). The phase-change layer 301 may be formed to athickness of 10 to 50 angstroms (Å). Alternatively, the thickness of thephase-change layer 301 may be thinner than 10 Å.

Subsequently, a second ILD 401 may be formed over the phase-change layer301 (FIG. 4). The second ILD 401 may be formed of a material with adifferent etch selectivity than the material of the first ILD 201, suchas any low thermal conductivity dielectric, including SiO₂, SiON,silicon nitride Si₃N₄, and fluorine doped SiO₂, that is different thanthe first ILD 201.

Next, a cell area 501 may be formed by removing portions of the secondILD 401, the phase-change layer 301, and the first ILD 201, asillustrated in FIG. 5. The cell area 501 may be formed according to anyprocess, such as a contact-etch process with one or more steps, that isable to remove the three layers. The cell area 501 may be formed to adepth of 100 to 200 nanometers (nm) and a width of 30 to 60 nm.

Adverting to FIG. 6, a heater liner 601 may be conformally depositedover the second ILD 401 and within the cell area 501. The heater liner601 may be formed of tungsten (W), titanium (Ti), or TiN. The heaterliner 601 may be formed to a thickness of 10 to 50 Å. However, thethickness of the heater liner 601 also may be thinner than 10 Å. Thethickness of the heater liner 601 may be the same as, or different than,the thickness of the phase-change layer 301.

The remaining area of the cell area 501 may be subsequently filled withan interlayer dielectric material to form an interlayer dielectricstructure 701 (FIG. 7). The interlayer dielectric material used to formthe interlayer dielectric structure 701 may be any interlayerdielectric, such as SiO₂, SiON, Si₃N₄, and fluorine doped SiO₂, providedthat the interlayer dielectric material employed differs from thedielectric material of the second ILD 401.

Subsequently, the portions of the heater liner 601 and the interlayerdielectric structure 701 above the top surface of the second ILD 401 maybe removed, as illustrated in FIG. 8. The portions of the heater liner601 and the interlayer dielectric structure 701 may be removed accordingto any known process, such as chemical mechanical polishing (CMP) downto the second ILD 401.

After the second ILD 401 is exposed, the second ILD 401 may be removedaccording to any process that is selective to the interlayer dielectricstructure 701, such as a blanket etch process. Additionally, a portionof the heater liner 601 surrounding the interlayer dielectric structure701 that is above the top surface of the phase change layer 301 isremoved, exposing a protruding portion 901 of the interlayer dielectricstructure 701, as illustrated in FIG. 9. The portion of the heater liner601 may be removed according to any process, such as by the one used toremove the second ILD 401 or another etch process. The remaining portionof the heater liner 601 forms a u-shaped heater liner 903. Asillustrated in FIG. 9, the top surface of the vertical portions of theu-shaped heater liner 903 may be coplanar with the top surface of thephase-change layer 301.

Adverting to FIG. 10, a dielectric spacer 1001 may be formed surroundingthe protruding portion 901 of the interlayer dielectric structure 701.The dielectric spacer 1001 may be formed of a dielectric material, suchas SiO₂, SiON, or Si₃N₄. Further, the portion of the phase-change layer301 that is not covered by the dielectric spacer 1001 may be removedleaving a covered phase-change layer 1003 between the dielectric spacer1001 and the first ILD 201 in a vertical direction, and on side surfacesof the exposed portion of the u-shaped heater liner 903 in a horizontaldirection.

Subsequently, an electrode 1101 may be formed over the entire structure,including the first ILD 201, the dielectric spacer 1001, and theinterlayer dielectric structure 701, as illustrated in FIG. 11. Theelectrode 1101 may be formed of Al, Pt, TiN, multilayer films of TiN/Ti,Ru, Ni, or polysilicon. The resulting structure is a PCRAM with a lowreset current of 0.1 to 10 nanoamp (nA)/nm² with a heating capacity of600 to 800° C.

Adverting to FIG. 12, a method of forming a PCRAM with a low resetcurrent, in accordance with another exemplary embodiment, begins withthe electrode 101, the first ILD 201 of a first interlayer dielectricmaterial, and the second ILD 401 of a second dielectric interlayerdielectric material, similar to the method of the first embodiment.However, in accordance with the alternative exemplary embodiment thephase-change layer 301 is excluded between the first ILD 201 and thesecond ILD 401.

Next, a cell area 1301 may be formed by removing portions of the secondILD 401 and the first ILD 201, as illustrated in FIG. 13. The cell area1301 may be formed according to any process, such as by a contact-etchprocess, as discussed above with respect to the cell area 501. The cellarea 1301 may be formed to a depth of 100 to 200 nm with a width of 30to 60 nm.

Adverting to FIG. 14, a heater liner 1401 may be conformally depositedover the second ILD 401 and within the cell area 1301. The heater liner1401 may be formed of W, Ti, or TiN and to a thickness of 10 to 50 Å.However, the thickness of the heater liner 1401 may be thinner than 10Å.

The remaining area of the cell area 1301 may be subsequently filled withan interlayer dielectric material to form an interlayer dielectricstructure 1501 (FIG. 15). The interlayer dielectric material used toform the interlayer dielectric structure 1501 may be any interlayerdielectric, such as SiO₂, SiON, Si₃N₄, or fluorine doped SiO₂, as longas the interlayer dielectric material used is different than thematerial of the second ILD 401.

Next, the portions of the heater liner 1401 and the interlayerdielectric structure 1501 above the top surface of the second ILD 401may be removed, as illustrated in FIG. 16. The portions of the heaterliner 1401 and the interlayer dielectric structure 1501 may be removedaccording to any known process, such as CMP down to second ILD 401.

After the second ILD 401 is exposed, it may be removed according to anyprocess that is selective to the interlayer dielectric material of theinterlayer dielectric structure 1501, such as a blanket etch process.Additionally, a portion of the heater liner 1401 that is above the topsurface of the first ILD 201 may be removed, exposing a protrudingportion 1701 of the interlayer dielectric structure 1501, as illustratedin FIG. 17. The portion of the heater liner 1401 may be removed by thesame etch process used to remove the second ILD 401 or by another etchprocess. The remaining portion of the heater liner 1401 forms a u-shapedheater liner 1703. As illustrated in FIG. 17, the top surface of thevertical portions of the u-shaped heater liner 1703 may be coplanar withthe top surface of the first ILD 201.

Adverting to FIG. 18, a phase-change layer 1801 may be formedsurrounding the interlayer dielectric structure 1501. The phase-changelayer 1801 may be formed by any process, such as by conformally formingphase-change material over the first ILD 201 and the interlayerdielectric structure 701 and subsequently removing the horizontalportions of the phase-change layer 1801 above both the first ILD 201 andthe interlayer dielectric structure 701, thereby forming thephase-change layer 1801 in the shape of thin spacers. A top surface ofthe phase-change layer 1801 may be coplanar with a top surface of theinterlayer dielectric structure 1501. The phase-change layer 1801 may beformed of any type of phase-change material, such as GST or AIST and maybe formed to a thickness of 10 to 50 Å. Alternatively, the thickness ofthe phase-change layer 1801 may be thinner than 10 Å. Subsequently, adielectric spacer 1803, such as of SiO₂, SiON, or Si₃N₄, may be formedsurrounding the phase-change layer 1801 around the protruding portion1701 of the interlayer dielectric structure 1501. The dielectric spacer1803 may be formed to a shorter height than the phase-change layer 1801and the interlayer dielectric structure 1501, as illustrated in FIGS. 18and 19, to expose the phase-change layer 1801.

Next, an electrode 1101 may be formed over the entire structure,including the first ILD 201, the dielectric spacer 1803, and theinterlayer dielectric structure 1501, as illustrated in FIG. 19. Theelectrode 1101 may be formed of Al, Pt, TiN, TiN/Ti, Ru, Ni, orpolysilicon.

Alternatively, FIG. 20 illustrates a method of forming a PCRAM with alow reset current, in accordance with another exemplary embodiment.After removing the second ILD 401 and the portion of the heater liner1401 that is above the top surface of the first ILD 201, exposing aprotruding portion 1701 of the interlayer dielectric structure 1501, asdiscussed above with respect to FIG. 17, a phase-change layer 2001 maybe formed surrounding the interlayer dielectric structure 1501. Thephase-change layer 2001 may be formed by conformally depositing thephase-change layer 2001 over the first ILD 201 and the interlayerdielectric structure 1501. The phase-change layer 2001 may be formed ofany type of phase-change material, such as GST or AIST and may be formedto a thickness of 10 to 50 Å. Alternatively, the thickness of thephase-change layer 2001 may be thinner than 10 Å.

Subsequently, a dielectric spacer 2003 may be formed on and surroundingthe phase-change layer 2001 around the protruding portion 1701 of theinterlayer dielectric structure 1501. The dielectric spacer 2003 may beformed of a dielectric material, such as SiO₂, SiON, or Si₃N₄. Uponformation of the dielectric spacer 2003, the portions of thephase-change layer 2001 that are not covered by the dielectric spacer2003 may be removed, such as above the protruding portion 1701 of theinterlayer dielectric structure 1501 and on either edge of thedielectric spacer 2003 above the first ILD 201. As discussed above, thedielectric spacer 2003 may be formed to a lesser height than thephase-change layer 2001 and the interlayer dielectric structure 1501, asillustrated in FIGS. 20 and 21, to expose the phase-change layer 2001.Alternatively, the top surfaces of the dielectric spacer 2003, thephase-change layer 2001, and the interlayer dielectric structure 1501may be coplanar (not shown for illustrative convenience).

Next, an electrode 1101 may be formed over the entire structure,including the first ILD 201, the dielectric spacer 2003, and theinterlayer dielectric structure 1501, as illustrated in FIG. 21. Theelectrode 1101 may be formed of Al, Pt, TiN, TiN/Ti, Ru, Ni, orpolysilicon.

In alternative embodiments, the locations of the phase-change layers andthe heater liner may be switched. Thus, in FIG. 21, feature 1703 may bethe phase-change layer and feature 2001 may be the heater liner. In FIG.19, feature 1703 may be the phase-change layer and feature 1801 may bethe heater liner. Further, in FIG. 11, feature 903 may be thephase-change layer and feature 1003 may be the heater liner.

The embodiments of the present disclosure achieve several technicaleffects, including PCRAM structures with higher heating capacities atlower reset currents. Embodiments of the present disclosure enjoyutility in various industrial applications as, for example,microprocessors, smart phones, mobile phones, cellular handsets, set-topboxes, DVD recorders and players, automotive navigation, printers andperipherals, networking and telecom equipment, gaming systems, anddigital cameras. The present disclosure therefore enjoys industrialapplicability in any of various types of highly integrated semiconductordevices.

In the preceding description, the present disclosure is described withreference to specifically exemplary embodiments thereof. It will,however, be evident that various modifications and changes may be madethereto without departing from the broader spirit and scope of thepresent disclosure, as set forth in the claims. The specification anddrawings are, accordingly, to be regarded as illustrative and not asrestrictive. It is understood that the present disclosure is capable ofusing various other combinations and embodiments and is capable of anychanges or modifications within the scope of the inventive concept asexpressed herein.

1. A method comprising: forming a first interlayer dielectric (ILD) overa bottom electrode; removing a portion of the first ILD, forming a cellarea; forming a u-shaped heater liner within the cell area; forming aninterlayer dielectric structure within the u-shaped heater liner, theinterlayer dielectric structure including a protruding portion extendingabove a top surface of the first ILD; forming a phase-change layer onside surfaces of the protruding portion and/or on the first ILDsurrounding the protruding portion; and forming a dielectric spacersurrounding the protruding portion.
 2. A method according to claim 1,comprising: forming the phase-change layer on the first ILD; andremoving a portion of the phase-change layer vertically contiguous withthe portion of the first ILD, prior to forming the u-shaped heaterliner.
 3. A method according to claim 2, comprising: forming theu-shaped heater liner within the cell area and between the phase-changelayer and the protruding portion with a top surface of the u-shapedheater liner being coplanar with a top surface of the phase-changelayer.
 4. A method according to claim 1, comprising: forming thephase-change layer on the first ILD; forming the dielectric spacer onthe phase-change layer; and removing a portion of the phase-change layerfrom the first ILD not under the dielectric spacer.
 5. A methodaccording to claim 1, further comprising: forming a second ILD over thephase-change layer; and forming the protruding portion to a height thatis coplanar with a top surface of the second ILD.
 6. A method accordingto claim 1, further comprising: forming a second ILD over the first ILDprior to forming the phase-change layer; and removing a portion of thesecond ILD vertically contiguous with the portion of the first ILD,prior to forming the u-shaped heater liner.
 7. A method according toclaim 1, comprising: forming the phase-change layer on side surfaces ofthe protruding portion; and forming the dielectric spacer surroundingthe phase-change layer.
 8. A method according to claim 1, comprising:forming the phase-change layer on side surfaces of the protrudingportion and on the first ILD; and forming the dielectric spacer abovethe phase-change layer on the first ILD and surrounding the phase-changelayer on the side surfaces of the protruding portion.
 9. A methodaccording to claim 1, further comprising forming a top electrode overthe first ILD, the dielectric spacer, and the protruding portion.
 10. Adevice comprising: a bottom electrode; a u-shaped heater liner; aninterlayer dielectric (ILD) on the bottom electrode, surrounding theu-shaped heater liner; an interlayer dielectric structure surrounded bythe u-shaped heater liner and including a protrusion extending above theu-shaped heater liner and a top surface of the ILD; a phase-change layeron the ILD and surrounding the interlayer dielectric structure and/or onside surfaces of the protrusion; a dielectric spacer above the ILD andsurrounding the protrusion; and a top electrode covering the ILD, theprotrusion, and the dielectric spacer.
 11. A device according to claim10, comprising the phase-change layer being on the ILD under thedielectric spacer.
 12. A device according to claim 11, comprising: a topsurface of the phase-change layer being level with a top surface of anexposed portion of the u-shaped heater liner above the ILD; and thephase-change layer surrounding the exposed portion of the u-shapedheater liner.
 13. A device according to claim 10, comprising: thephase-change layer being on side surfaces of the protrusion, between thedielectric spacer and the protrusion.
 14. A device according to claim13, comprising the phase-change layer being contiguous and aligned withvertical portions of the u-shaped heater liner.
 15. A device accordingto claim 13, wherein a height of the phase-change layer and a height ofthe protrusion are substantially the same.
 16. A device according toclaim 15, comprising the top surface of the phase-change layer extendingabove a top surface of the dielectric spacer.
 17. A device according toclaim 10, wherein a thickness of a heater liner layer forming theu-shaped heater liner is the same as a thickness of the phase-changelayer.
 18. A device according to claim 10, comprising: a first portionof the phase-change layer being on the ILD under the dielectric spacer;and a second portion of the phase-change layer being on side surfaces ofthe protrusion between the dielectric spacer and the protrusion.
 19. Adevice according to claim 18, comprising the first portion of thephase-change layer being contiguous and aligned with vertical portionsof the u-shaped heater liner.
 20. A method comprising: forming a firstinterlayer dielectric (ILD) over a bottom electrode; forming a secondILD on the first ILD; removing a portion of the first and second ILDs,forming a cavity; conformally depositing a heater liner over the secondILD and within the cavity; planarizing the heater liner down to thesecond ILD; filling the cavity with an interlayer dielectric material;removing a portion of the heater liner adjacent the second ILD and thesecond ILD, leaving a portion of the interlayer dielectric materialprotruding from the cavity; forming a phase-change layer on sidesurfaces of the protruding portion and/or on the first ILD surroundingthe protruding portion; forming a dielectric spacer surrounding theprotruding portion; and forming a top electrode over the first ILD, thedielectric spacer, and the protruding portion.